The global system for mobile communications (GSM) was designed as a second generation system for enabling voice communication to go wireless. However, customers have increasingly sought additional wireless services, such as text messaging and access to data networks. Therefore, third generation systems are designed to provide multimedia communication, including images and video. For example, enhanced data rates for GSM evolution (EDGE) can provide third generation services, including advanced features that improve spectrum efficiency. Likewise, the universal mobile telecommunications services (UMTS) provides flexible delivery of many different services. The UMTS terrestrial radio access network (UTRAN) includes network elements that handle radio-related functionality for UMTS.
By definition, handover means transfer of a user connection from one radio channel to another. One type of handover is an inter-RAT handover, when a user equipment (UE) moves to a different radio access technology (RAT). For example, when the user equipment (UE) reaches the end of a coverage area for UMTS services, it can handover to a 2G service like GSM (if the UE supports multiple RAT). An inter-RAT handover procedure can be initiated in a variety of ways. For instance, a radio network subsystem (RNS) might send a command explicitly telling the UE to move to a different RAT, or the UE might select a cell that belongs to a different RAT.
In the context of a general packet radio system (GPRS), rules for packet switched (PS) handover have been defined in Release 6 of the Third Generation Partnership Project (3GPP), based on GSM/EDGE Radio Access Network (GERAN) research and development. To support handover between radio access technologies (i.e. to support inter-RAT handovers), such as handovers between UMTS and GSM, signalling has been added in the radio resource control (RRC) with the Uu interface, and in the radio access network application part (RANAP) with the Iu interface. Pertinent details about GPRS can be found in 3GPP TS 48.018 V6.14.0 (2006-07), “BSS GPRS Protocol (BSSGP) (Release 6),” of which pages 52-58 and 106-150 are incorporated by reference herein in their entirety. Also, pertinent details about RANAP signalling can be found in 3GPP TS 25.413 V6.0.0 (2005-06), “UTRAN Iu interface RANAP signalling (Release 6),” of which pages 33-37 and 122-170 are incorporated by reference herein in their entirety.
To support inter-RAT PS handover from UTRAN to GERAN, a serving radio network controller (SRNC) needs system information such as packet system information (PSI/SI which is the same as SI/PSI) blocks of the target GERAN cell. The 3GPP group known as “RAN2” has added a choice structure information element (IE) in RRC handover from the UTRAN command message, in order to transfer these PSI/SI blocks to the user equipment (UE). The assumption about how the SRNC gets these blocks was to reuse a RAN Information Management (RIM) procedure with Network Assisted Cell Change (NACC).
Problematically, the RIM/NACC procedure is optional in the network. Consequently, there is no guarantee that the SRNC will get the PSI/SI blocks in time for the handover.
In the UMTS network, the SRNC may have to get the PSI/SI blocks from a drift radio network controller (DRNC) or controlling RNC (CRNC). Thus, the procedure may be even more complicated, more problematic, and less likely to provide the SRNC with the PSI/SI blocks in time.
The PSI/SI blocks are mandatory information for PS handover, whereas RIM/NACC is an optional feature for optimization. Thus, the criticality of missing PSI/SI blocks is higher in the PS handover than in the RIM/NACC procedure.